The present invention relates to a process for removing heavy metal cations and/or alkali metal cations from aqueous solutions with an ion exchanger material, which comprises a cation exchanger loaded with alkaline earth metal ions, and subsequently regenerating the loaded or exhausted ion exchanger material.
In many areas of chemical technology ion exchangers have been used successfully for the treatment of all types of saline aqueous solutions. The ion exchangers cannot destroy any pollutants. However, they can, for example, enrich pollutants of which there are only small concentrations in the solution, such as e.g., heavy metals. Furthermore, ion exchangers permit the replacement of ions that are not hygenically harmful but which prevent the process from operating efficiently or which make the process more difficult to operate by other ions, which do not have these properties.
As a rule, the chemical reactions on the ion exchangers are reversible. This means that the direction of the exchange can be reversed by the addition of suitable chemicals. As a result of this characteristic for regeneration, ion exchanger resins can be reused almost indefinitely. During regeneration, the pollutants that are only slightly concentrated in the original solution are freed from the exchanger and thereby become available in an eluate solution in concentrated form in which they can be more readily removed or further processed.
The state of the art for removing heavy metals from dilute aqueous solutions (e.g. raw water) is the use of cation exchangers that are conditioned, i.e., completely or partially loaded with sodium or calcium (LEWATIT.RTM. "Entfernung geloster Schwermetall-Spuren aus neutralisiertem Mischabwasser durch Lewatit-Ionenaustauscher, Technische Information [in translation, LEWATIT.RTM. "Removal of Dissolved Heavy Metal Traces from Neutralized Mixed Wastewater by means of LEWATIT Ion Exchangers" Technical Information], (1 Aug. 1981) Bayer AG), as illustrated in equation (1). EQU R=Na.sub.2 +Cu.sup.2 +.fwdarw.R=Cu+2Na.sup.+ ( 1)
Usually, the spent exchanger resin produced according to equation (1) is regenerated in two sequential, separate steps. In the first step, as shown in equation (2), the spent exchanger is converted into the free acid form with a strong mineral acid: EQU R=Cu+2HCl.fwdarw.R=H.sub.2 +CuCl.sub.2 ( 2)
Then, in the second step, as shown in equation (3), the exchanger resin is conditioned with an NaOH solution or a Ca(OH).sub.2 solution, that is, it is converted into the desired type of neutral salt: EQU R=H.sub.2 +2NaOH.fwdarw.R=Na.sub.2 +H.sub.2 O (3)
The result of the first regeneration step is that additional ions get into the wastewater, so that a quantity of salt that is larger than the quantity removed from the raw water in the work cycle has to be removed.
The state of the art for removing alkali metal ions is, for example, the reduction of the concentration of the alkali ions, such as sodium and potassium ions in aqueous sugar solutions, by means of cation exchangers, which are loaded with calcium or magnesium, as illustrated in equation (4). (F. Schneider: "Technologie des Zuckers", Verlag Schaper [in translation "Technology of Sugar", Schaper Publishing Company], Hannover, (1968), Chapter 10 "Ionenaustausch" [in translation "Ion Exchange"], in particular pages 609 to 629). EQU R=Mg+2Na.sup.+ .fwdarw.R=Na.sub.2 +Mg.sup.2+ ( 4)
In processes according to the state of the art, magnesium chloride is used for regenerating the spent exchanger, whereby the anions of the regenerating agent (here the chloride ions) do not take part in the regeneration reaction, as shown in equation (5), and can result in circumstances permitting a noticeable load of additional ions in the wastewater. EQU R=Na.sub.2 +MgCl.sub.2 .fwdarw.R=Mg+2NaCl (5)